Several conventional guided-wave acoustooptic spectrum analyzers are described on pages 1072 to 1098 of "IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS, VOL. CAS-26. No. Dec. 12, 1979". One of the guided-wave acoustooptic spectrum analyzers comprises a Ti-diffused LiNbO.sub.3 waveguide, a pair of waveguide lenses, a transducer for producing surface acoustic wave (simply called "SAW" hereinafter), a laser diode for radiating light, guiding of which is provided by butt-coupling of the laser diode to a substrate, and a photodetector array for detecting light which is deflected in response to the frequencies of the SAW.
In operation, the light radiated from the laser diode propagates through the Ti-diffused LiNbO.sub.3 waveguide, and beams thereof are made parallel by the first lens of the pair of the waveguide lenses. The light that has thus passed through the first lens is deflected by the SAW.
The extent of the deflection is responsive to the frequency of the SAW produced by the transducer. The light thus deflected is focused on the photodetector array so that the frequency spectrum of the light can be analyzed in real time in accordance with the position of the light receiving section of the photodetector array, which depends in turn upon the extent of deflection of the light.
According to the guided-wave acoustooptic spectrum analyzer described above, the frequency resolution .delta..sub.f is determined by the equation (1). ##EQU1## where V is a propagating velocity of the SAW, and W is a width of the light beams which are made parallel by the first lens.
In a case where a pulse signal is applied to the transducer, a detectable minimum value .tau..sub.min of the pulse width therein is determined by the equation (2). ##EQU2##
Therefore, the foregoing system has the disadvantage that the minimum value .tau..sub.min can not be smaller than a certain level, which depends upon the frequency resolution .delta..sub.f, since the resolution .delta..sub.f is proportional to the width W of the collimated light beam, while the minimum value .tau..sub.min of the pulse width is inversely proportional to the width W.
A further disadvantage is present in the guided-wave acoustooptic spectrum analyzer described above, namely that the availability of the SAW is not sufficient because the SAW which are propagated in a direction away from the light path between the pair of waveguide lenses is absorbed.
There is a still further disadvantage in the guided-wave acoustooptic spectrum analyzer described above, namely that the light is scattered by the Ti-diffused LiNbO.sub.3 waveguide, if electrodes of the transducer are provided in the light path between the pair of the waveguide lenses. This is in contrast to certain features of the invention to be described in detail later.